Speak of the Devil is the 14th main quest in Kingdom Come: Deliverance 2. Following the meeting at 
Suchdol, Henry and Zizka set out to locate “Dry Devil,” a local rebel leader who has been captured by Sigismund's forces.
This quest introduces you to the 
Kuttenberg outskirts and the game's early pistoles. It follows The Sword and the Quill.
The journey to Devil's Den
The quest begins on horseback. You will follow Zizka as he explains the plan to check local taverns for leads.
You can ride with him to enjoy the scenery or choose the option to fast-travel directly to the Devil's Den inn.
At Devil's Den
Before you approach Zizka at the inn to trigger the next sequence, ensure you are wearing your armor. Upon arrival, you will find a drunkard named Kubyenka being accosted by several armed men who accuse him of cheating at dice.
An unavoidable combat starts almost immediately after you encounter Kubyenka being thrown out of the tavern.
Once the attackers are solved, you are introduced to Innkeeper Treadlight who first takes offense but then calms down. You can then further question Kubyenka about him, and the whereabouts of Dry Devil.
Kubyenka reveals that Dry Devil is being held in Kuttenberg and is scheduled for a public execution at Sigismund's camp. The group decides to ambush the prisoner transport, but they first go to retrieve some pistoles to their old camp nearby.
Find the hand cannons
After the dialogue, Kubyenka immediately starts walking towards the horses, which is a point of common confusion, as at this point you can't see him on the main map.
Follow Kubyenka on horseback to the woods. He first stops to relieve himself, then you need to follow him on foot. You will arrive at the 
Old Campsite interesting site.
Once you arrive at the camp, Kubyenka's memory proves to be terrible, due to his heavy drinking, as he can't remember the exact location.
You will need to dig at several locations to find the gear. If you didn't bring a Spade, you can find one leaning against the leaf-covered tent at the camp. Sadly you can't skip ahead and have to follow Kubyenka through all fake sites until you reach the correct one.
First site
Digging here reveals only a bottle of 
Saviour Schnapps and puts a Deer Hunting Spot on your map.
Second site (the river)
Kubyenka asks you to retrieve some Moonshine from a basket submerged in the creek. Giving it to him makes him noticeably more intoxicated on the way back but it doesn't negatively impact the quest progression.
Third site
You will accidentally exhume an old friend of Kubyenka's. You can loot the body for some not-great armor, a Silver Badge of Defense, a Die of Misfortune, and some lockpicks.
Fourth site
This is the correct location. You will find the Carefully Wrapped Handgonnes, ammunition, a Gunsmith's Kit, and a Strong Bowman's Brew.
Collect the weapons and follow Kubyenka to the designated ambush spot where Zizka is waiting with a roadblock.
Save Zizka: caravan ambush fight
Speak to Zizka to begin the ambush. He will first ask you how it went with Kubyenka. Complaining doesn't cost you any reputation.
A cutscene triggers where the group attempts to use the hand cannons to scare the guards. The results are laughably bad (only one horse goes down), and somebody has even shot Dry Devil's leg. Then, you are forced into a confrontation with the heavily armed escort.
The confrontation starts with a negotiation, but you cannot evade the upcoming fight. However, you can influence the strength of both sides.
When the lead guard calls your bluff, you have one chance to resolve the situation via a Skill Check (Charisma, Intimidation, or Speech).
The difficulty: These checks are extremely high (Hard/Very Hard). Even a Charisma stat of 25 may fail.
Charisma (almost impossible): Even at high level you are very improbable to pass the Charisma check. Even if you win, there is no difference, but if you lose, several of your men will run away. This option is not recommended.
Speech & Intimidation (easier): Passing Speech or Intimidation will cause some of the enemy guards to run away, making the subsequent fight much easier.
How to save Zizka: combat help
Regardless of the dialogue outcome, a fight is almost guaranteed. These soldiers are well-armored and highly skilled.
Use the handgonne: You start the fight with a pre-loaded hand cannon. Use it immediately to take out the soldier closest to Zizka so he can move further.
After this, we recommend using the sword if you are good with Master Strikes. Very soon, their leader will start coming after you. Mutt and Zizka can distract him well enough to get a killing blow.
Quickly loot him to not miss out, he has some great armor including Magdeburg Cuirass (very good chest plate with 96 Strength at full durability) and Saxon plate legs.
Use the cart as an obstacle to prevent the guards from surrounding you. Don't forget to utilize Mutt to distract the enemies (preferably the most armored one) while you deal with the lower-level guards.
After the fight
Talk to Zizka to teleport back to the Devil's Den with the rescued Dry Devil.
At the inn, the group will discuss their next moves. Completing this quest opens up the Kuttenberg region for free exploration and begins two branching main quests: The Devil's Pack and Into the Underworld. It is recommended to pursue The Devil's Pack first to reunite the rest of the band.
However, if you aim for 100% Completion, go immediately complete the extremely missable task 
Arrow-Head.
You now have a bed at Devil's Den. There is no need to talk to Innkeeper Treadlight, even if Zizka told you so, but he does have introductory dialogue which rewards reputation.
Next quest: The Devil's Pack or Into the Underworld or Arrow-Head.
Speak of the Devil FAQ
What main quest number is Speak of the Devil?
It is the 14th main quest in Kingdom Come: Deliverance 2. It follows The Sword and the Quill and introduces Kuttenberg outskirts and early pistoles.
Where do you find the handgonnes?
Follow Kubyenka from Devil's Den to the Old Campsite. Dig at the fourth site to get the Carefully Wrapped Handgonnes and related gear.
Can you skip the caravan fight?
No. Negotiation can thin the enemy with Speech or Intimidation, but a fight is almost guaranteed.
Which ambush dialogue checks help?
Speech and Intimidation can make some enemy guards flee. Charisma is very hard and is not recommended.
What should you do after this quest for 100%?
Complete the missable Arrow-Head task soon if you track full completion.
What main quests follow Speak of the Devil?
The Devil's Pack and Into the Underworld branch open.